The present invention refers to a method for producing a blank of titanium-doped glass having a high silicic acid content and an internal transmission of at least 60% in the wavelength range of 400 nm to 700 nm at a sample thickness of 10 mm, and with a given fluorine content for use in extreme ultraviolet (EUV) lithography.
Furthermore, the present invention refers to a blank of titanium-doped silica glass for use in EUV lithography.
In EUV lithography, highly integrated structures having a line width of less than 50 nm are produced by microlithographic projection devices. Radiation from the EUV range (extreme ultraviolet light, also called soft X-ray radiation) with wavelengths of about 13 nm is used. The projection devices are equipped with mirror elements which consist of titania-doped glass having a high silicic-acid content (hereinafter also called “TiO2—SiO2 glass” or “Ti-doped silica glass”) and which are provided with a reflective layer system. These materials are distinguished by an extremely low linear coefficient of thermal expansion (shortly called “CTE”; coefficient of thermal expansion), which is adjustable through the concentration of titanium. Standard titania concentrations are between 6% by wt. and 9% by wt.
In the intended use of such blanks, which are made from synthetic titanium-doped glass having a high silicic-acid content, as a mirror substrate, the upper side thereof is provided with a reflective coating. The maximum (theoretical) reflectivity of such an EUV mirror element is about 70%, so that at least 30% of the radiation energy is absorbed in the coating or in the near-surface layer of the mirror substrate and converted into heat. In the volume of the mirror substrate, this leads to an inhomogeneous temperature distribution with temperature differences that, according to information given in the literature, may amount to 50° C.
Therefore, it would be desirable to provide a deformation that is as small as possible, if the glass of the mirror substrate blank had a CTE which is zero over the whole temperature range of the working temperatures arising during use. In Ti-doped silica glasses, however, the temperature range with a CTE of approximately zero can in actual fact be very narrow.
The temperature at which the coefficient of thermal expansion of the glass equals zero shall also be called zero crossing temperature or TZC (temperature of zero crossing) hereinafter. The titanium concentration is normally set such that a CTE of zero is obtained in the temperature range between 20° C. and 45° C. Volume regions of the mirror substrate with a higher or lower temperature than the preset TZC expand or contract so that, despite an altogether low CTE of the TiO2—SiO2 glass, deformations may arise that are detrimental to the imaging quality of the mirror.
In addition, the fictive temperature of the glass plays a role. The fictive temperature is a glass property that represents the state of order of the “frozen” glass network. A higher fictive temperature of the TiO2—SiO2 glass is accompanied by a lower state of order of the glass structure and by a greater deviation from the most energetically advantageous structural arrangement.
The fictive temperature is influenced by the thermal history of the glass, especially by the last cooling process. In the last cooling process, different conditions are bound to prevail for near-surface regions of a glass block as compared with the central regions, so that different volume regions of the mirror substrate blank already have different fictive temperatures due to their different thermal history. The fictive temperatures, in turn, correlate with correspondingly inhomogeneous regions with respect to the CTE curve. In addition, however, the fictive temperature is also influenced by the amount of fluorine, because fluorine has an impact on structural relaxation. Fluorine doping permits the adjustment of a low fictive temperature and, consequently, also a smaller slope of the CTE curve with respect to the temperature.
The Ti-doped silica glass is produced by flame hydrolysis, starting from precursor substances containing silicon and titanium. First of all, a porous soot body of titanium-doped SiO2 is produced, which is vitrified into a dense glass body. Optionally, the soot body is subjected, prior to vitrification, to a drying process (e.g. by treatment in a halogen-containing atmosphere) for reducing the hydroxyl group content (OH group content). Doping with titanium oxide, however, leads to a brownish appearance or staining of the glass due to a more or less strong concentration of Ti3+ ions in the glass matrix. The shaped bodies for this application, hereinafter also called blanks, are large, dark-brown plates with dimensions of up to about 70×60×20 cm3. The blanks must be checked with respect to their optical properties and with respect to defects or inhomogeneity due to the manufacturing process. The brownish appearance of the glass has turned out to be problematic, since common optical measuring methods that presuppose transparence in the visible spectral range can only be used to a limited degree or cannot be applied at all.
The literature has proposed various solutions for limiting the amount of Ti3+ ions in favor of Ti4+ ions by way of an oxidation treatment. When a Ti-doped silica glass is used with a relatively high hydroxyl group content, the OH groups permit the desired oxidation of Ti3+ into Ti4+. This is, for example, described for Ti-doped silica glass by Carson and Mauer in “Optical Attenuation in Titania-Silica Glasses,” J. Non-Crystalline Solids, Vol. 11(1973), pp. 368-380, which indicates a reaction according to formula 2Ti3++2OH−→2Ti4++2O2−+H2.
This procedure is adopted in European Patent Application Publication No. EP 2 428 488 A1, particularly with respect to optimized conditions for the process of oxidation and the out-diffusion of hydrogen during an annealing treatment. The Ti-doped silica glass disclosed in EP 2 428 488 A1 is not doped with fluorine, has a high OH content of more than 600 wt. ppm, and has a relatively low hydrogen content (less than 2×1017 molecules/cm3). To ensure a high OH content, the addition of water vapor during the deposition process is recommended. A two-stage deposition process is described in which TiO2—SiO2 soot particles are first formed that are subsequently consolidated and vitrified. A one-stage process is also described in which the soot particles are vitrified immediately (so-called “direct quartz” or “DQ method”). The amount of Ti3+ ions in the Ti-doped silica glass is disclosed as being less than 3 ppm and the internal transmission over a wavelength range of 340 nm to 840 nm is disclosed as being greater than 90%. However, no information is provided about the thickness of the sample.
International Application Publication No. WO 2004/089836 A1 discloses a Ti-doped silica glass with a fluorine doping that exhibits a very flat slope of the coefficient of thermal expansion over a relatively wide temperature range. First, the porous TiO2—SiO2 soot body is predried in air at 1200° C., which entails a first reduction of the OH content and an oxidation of Ti3+ ions. Subsequently, for fluorine doping, the TiO2—SiO2 soot body is exposed to an atmosphere with 10% by vol. of SiF4 in oxygen or in helium for several hours. Apart from fluorine doping, this treatment entails a further reduction of the OH content. To prevent a dark coloration or staining during vitrification of the soot body, it is suggested in WO 2004/089836 A1 that the soot body should be treated prior to vitrification in an oxygen atmosphere for several hours in the temperature range between 300° C. and 1300° C., before the subsequent vitrification step is carried out under helium. The glass body of fluorine- and titanium-doped silica glass is then shaped into a blank and subjected to an annealing treatment for setting the fictive temperature. Information on the amount of Ti3+ ions or on the dark staining or on the internal transmission is not provided in WO 2004/089836 A1.
International Application Publication No. WO 2006/004169 A1 resumes the examples of WO 2004/089836 A1 with respect to the amount of Ti3+ and the information on the internal transmission. The method according to WO 2006/004169 A1 also provides an oxygen treatment of a TiO2—SiO2 soot body with fluorine doping prior to vitrification (under helium.). Fluorine doping is carried out in an atmosphere containing oxygen and fluorine. The Ti-doped silica glass produced in this manner contains 10 wt. ppm OH groups and 12 wt. ppm Ti3+ ions. The fluorine content is 120 wt. ppm and 6,300 wt. ppm, respectively. The internal transmission in the wavelength range of 400 nm to 700 nm is specified to be more than 80% in the case of this relatively high content of Ti3+. However, this is for a glass thickness of only one millimeter. Converted to a sample with a thickness of 10 mm, this corresponds to a value for the internal transmission of only 10%.
The methods according to WO 2004/089836 A1 and WO 2006/004169 A1 are very complicated technically and do not yield an acceptable, sufficiently high internal transmission for realistic sample thicknesses in the range of 10 mm.
It is known from U.S. Patent Application Publication No. 2006/0179879 A1 that in a TiO2—SiO2 glass for use in EUV lithography, the CTE curve over the temperature obtained in the course of operation can be influenced, apart from a homogeneous distribution of the titanium concentration, by further parameters (e.g., by doping with fluorine and by the OH content). Fluorine may also serve as a drying reagent with which the OH content can be set to less than 100 ppm. Inversely, an OH content of up to 1500 ppm is achieved through the action of water vapor during vitrification. In one embodiment according to US 2006/0179879 A1, a fluorine-doped TiO2—SiO2 soot body is obtained by flame hydrolysis of precursor substances containing silicon, titanium and fluorine. In a subsequent process step, the soot body is vitrified or consolidated in an inert gas atmosphere containing water vapor. The fluorine content of the TiO2—SiO2 glass is in the range of 500 wt. pm to 2000 wt. ppm. No information is provided on the amount of the Ti3+ ions in the TiO2—SiO2 glass, on the OH content and on the internal transmission in the visible wavelength range of this fluorine-doped TiO2—SiO2 glass.
Apart from the aforementioned embodiment, US 2006/0179879 A1 also generally discusses the production of quartz glass according to the so-called soot method under the heading “Soot Formation Followed by Consolidation.” Thus, the SiO2 soot body can be subjected, at about 1000° C., to a treatment with helium, hydrogen, water vapor or a doping gas, such as CF4, in the case of a desired fluorine doping, before a subsequent vitrification step is carried out at a higher temperature. No information is provided on the impacts of the treatment with helium, hydrogen or water vapor on the SiO2 soot body or on the sintered quartz glass. However, it must be assumed that, since a drying of the soot body prior to vitrification is obviously not intended, a high OH content is present in the soot body, which might even rise due to the water vapor-containing atmosphere and will lead to undesired bubbles during vitrification.
International Application Publication No. WO 2009/084717 A1, U.S. Patent Application Publication Nos. 2010/0179047 A1 and 2014/0155246 A1 and European Patent Application Publication No. 2 377 826 A1 are additional prior art publications regarding TiO2—SiO2 glass with fluorine-codoping.
In sum, according to the prior art, the reduction of Ti3+ ions in favor of T4+ ions in Ti-doped silica glass is ensured either by a sufficiently great amount of OH groups, whereby an internal oxidation with hydrogen diffusing out occurs, or at a low OH group content, wherein an oxygen treatment is required prior to vitrification. Such oxygen treatment demands a high treatment temperature and special corrosion-resistant furnaces, and is thus expensive.
In TiO2—SiO2 glass with F-codoping, the problems regarding brown coloration or staining caused by a high amount of Ti3+ ions are especially significant because, due to fluorine, there are virtually no OH groups present which may induce an oxidation of Ti3+ into Ti4+.
Moreover, it has been found that although the known oxygen treatment prior to vitrification reliably increases the amount of oxygen, whereby oxidation occurs once in favor of Ti4+ ions, this measure is not permanent, for instance, when the vitrified blank is shaped under a reducing atmosphere (e.g. by applying an oxyhydrogen flame adjusted in a reducing manner). This means that, due to the oxygen treatment, the oxygen is available only once for the oxidation of Ti3+ to Ti4+, so that under reducing conditions, Ti3+ ions are increasingly formed again, which is known to lead to the dark appearance or staining of the glass.